1. Field of the Invention
The present invention relates to a GaN-based semiconductor light-emitting element such as a light-emitting diode or a laser diode that operates at wavelengths over the ultraviolet range and the entire visible radiation range, which covers blue, green, orange, and white parts of the spectrum. Such a light-emitting element is expected to be applied to various fields of technologies including display, illumination, and optical information processing.
2. Description of the Related Art
A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element because its bandgap is sufficiently wide. Among others, gallium nitride-based compound semiconductors including Ga as a Group III element (which are hereinafter referred to as “GaN-based semiconductors” and are represented by the formula AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1)) have been researched extensively. As a result, blue light-emitting diodes (LEDs), green LEDs, and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four fundamental vectors a1, a2, a3, and c of a wurtzite crystal structure. The fundamental vector c runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. A plane terminated by a Group III element such as Ga is called “+c-plane” or “(0001) plane” whereas a plane terminated by a Group V element such as nitrogen is called “−c-plane” or “(000-1) plane”, and the two are discriminated from each other. It should be noted that the “c-axis” and the “c-plane” are sometimes referred to as “C-axis” and “C-plane”.
In fabricating a semiconductor element using GaN-based semiconductors, a c-plane substrate, i.e., a substrate of which the principal surface is a (0001) plane, is used as a substrate on which GaN-based semiconductor crystals will be grown. In a c-plane, however, Ga atoms and nitrogen atoms are not on the same atomic plane, thus producing electrical polarization. That is why the “c-plane” is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated in the InGaN quantum well of the active layer in the c-axis direction. Once such a piezoelectric field has been generated in the active layer, some positional deviation occurs in the distributions of electrons and holes in the active layer. Consequently, due to the quantum confinement Stark effect of carriers, the internal quantum yield decreases, thus increasing the threshold current in a semiconductor laser diode and increasing the power dissipation and decreasing the luminous efficacy in an LED. Meanwhile, as the density of injected carriers increases, the piezoelectric field is screened and also the emission wavelength is varied.
Thus, to overcome these problems, it has been proposed that a substrate of which the principal surface is a non-polar plane such as a (10-10) plane that is perpendicular to the [10-10] direction and that is called an “m-plane” (m-plane GaN-based substrate) be used. As used herein, “-” attached on the left-hand side of a Miller index in the parentheses means a “bar”. As shown in FIG. 2, the m-plane is parallel to the c-axis (i.e., the fundamental vector c) and intersects with the c-plane at right angles. On the m-plane, Ga atoms and nitrogen atoms are on the same atomic-plane. For that reason, no electrical polarization is produced perpendicularly to the m-plane. That is why if a semiconductor multilayer structure is formed perpendicularly to the m-plane, no piezoelectric field is generated in the active layer, thus overcoming the problems described above. The “m-plane” is a generic term that collectively refers to a family of planes including (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes.
Nitride semiconductor light-emitting elements that have a non-polar plane such as an m-plane or a semi-polar plane as the principal surface are known to emit polarized light. For instance, Japanese Laid-Open Patent Publication No. 2009-38292 proposes a structure that maintains the optical polarization characteristics of light emitted from a nitride semiconductor light-emitting element that has a non-polar plane or a semi-polar plane as the principal surface through the random placement of resin molecules. FIG. 3A is a sectional view illustrating the structure that is disclosed in Japanese Laid-Open Patent Publication No. 2009-38292. In FIG. 3A, a light-emitting element 302 which emits optically polarized light is put on a package base 301 and a light-transmissive resin portion 303 is disposed so as to surround the light-emitting element 302. The light-transmissive resin portion 303 which has a disordered structure does not exhibit birefringence.
An example of a mode of a resin sealing portion for a nitride semiconductor light-emitting element can be found in Japanese Patent Translation Publication No. Hei 11-500584, which proposes a semiconductor light-emitting element structure that uses a luminous substance pigment to emit white light. FIG. 3B is a sectional view illustrating the structure that is disclosed in Japanese Patent Translation Publication No. Hei 11-500584. In the semiconductor light-emitting element illustrated in FIG. 3B, a semiconductor base 305 is put in a container 304 and a casting material 306 is disposed so as to surround the semiconductor base 305. The casting material 306 contains luminous substance pigments 307 which convert light emitted from the semiconductor base 305 into long-wave light.
Another example of a mode of a resin sealing portion for a nitride semiconductor light-emitting element can be found in Japanese Laid-Open Patent Publication No. 2005-197317, which discloses a structure that raises the light emission efficiency by enhancing the refractive index of a medium. FIG. 3C is a sectional view illustrating the structure that is disclosed in Japanese Laid-Open Patent Publication No. 2005-197317. In FIG. 3C, an optical semiconductor element 309 is put in a package 308 and a medium 310 is disposed so as to surround the optical semiconductor element 309. The medium 310 contains nanoparticles 312 made from a material that has, when in a bulk state, a refractive index higher than that on an exit surface of the optical semiconductor element 309 within the wavelength range of emitted light. The medium 310 also contains fluorescent particles 311 which convert emitted light of the optical semiconductor element 309 into long-wave light.
However, when a light-emitting element that has optical polarization characteristics is a light source, the amount of reflection at a surface of an object varies and how the object looks accordingly varies, depending on the direction of optical polarization, namely, the direction in which the light-emitting element is set up. This problem is caused by a difference in reflectance between P polarization and S polarization (the reflectance of P polarization is higher than that of S polarization). While improving the optical polarization degree is important in applications where optical polarization characteristics are utilized without modification, having optical polarization characteristics thus impairs the performance of a light-emitting element in general illumination uses.
Another problem is that, because it is the nature of light to travel in a direction perpendicular to the optical polarization direction, optically polarized light that is generated by a nitride semiconductor light-emitting element deviates from radiant intensity distribution characteristics having the pattern of Lambert's cosine law (Lambertian/Lambert distribution).
These problems are particularly noticeable in gallium nitride-based light-emitting elements that have a non-polar plane or a semi-polar plane as the principal surface, and pose a major obstacle to putting light-emitting elements that have a non-polar plane or a semi-polar plane as the principal surface into practice.
Japanese Laid-Open Patent Publication No. 2009-38292, where the objective is to maintain the optical polarization characteristics of a nitride semiconductor light-emitting element that has an m-plane as the principal surface, is not a solution to the above-mentioned problems. Japanese Patent Translation Publication No. Hei 11-500584 and Japanese Laid-Open Patent Publication No. 2005-197317, too, do not provide a solution to the above-mentioned problems because these publications are not about nitride semiconductor light-emitting elements having optical polarization characteristics in the first place.